Ophthalmic lens elements are frequently coated to achieve special properties. Anti-reflection coatings improve the transmittance of visible light and the cosmetic appearance of the lenses. Reflective coatings may be employed in sun lenses to reduce light transmittance to the eye, to protect the eye from UV radiation and/or to impart cosmetic colorations to the lens. Coatings may also provide other beneficial properties such as increased hardness and scratch resistance and anti-static properties.
Desirable lens coatings may be created by applying single or multiple layers of metal or semi-metal oxides to surfaces of the lens element. Such materials include oxides of silicon, zirconium, titanium, neobium and tantalum. Examples of such multilayer coatings are given, for example, in U.S. Pat. No. 5,719,705 to Machol entitled "Anti-static Anti-reflection Coatings", assigned to applicant. Interference filter coatings for sunglasses are disclosed, for example, in U.S. Pat. No. 2,758,510 to Auwarter.
Various methods are disclosed in the prior art for applying metal and semi-metal oxide coatings to ophthalmic lenses. Ritter et al. U.S. Pat. No. 4,172,156 discloses vacuum evaporation in an oxygen atmosphere of Cr and Si to form coating layers on a plastic lens. Reactive sputter deposition of various oxide layers on lens elements is disclosed in the above-mentioned '705 patent to Machol.
Reactive sputtering in general is a conventional technique often used, for example, in providing thin oxide coatings for such items as semi-conductor wafers or glass lamp reflectors. Examples of various conventional vacuum deposition systems for the formation of coatings by reactive sputtering are disclosed in the following patents: U.S. Pat. Nos. 5,616,224 to Boling; U.S. Pat. No. 4,851,095 to Scobey et al.; U.S. Pat. No. 4,591,418 to Snyder; U.S. Pat. No. 4,420,385 to Hartsough; British Patent Application GB 2,180,262 to Wort et al.; Japanese Kokai No. 62-284076 to Ito; and German Patent No. 123,714 to Heisig et al.
Most ophthalmic lenses produced today are made from a single plastic body or laminated plastic wafers. The plastic material may include thermoplastic material such as polycarbonate or thermoset material such as diallyl glycol carbonate types, e.g. CR-39 (PPG Industries). The material may also be a cross linkable polymeric casting composition such as described in U.S. Pat. No. 5,502,139 to Toh et al and assigned to applicant. The challenge is to adapt conventional vacuum deposition systems to high volume production of plastic lens elements, while ensuing a high degree of control over the thickness and composition of the coating.
Accordingly, it is an object of the present invention to improve the degree of control over the thickness and composition of thin metal and semi-metal oxide coatings deposited on plastic lenses.
Anti-reflection coating of plastic ophthalmic lenses by physical vapor deposition has traditionally been performed by means of thermal evaporation, or more recently, e-beam evaporation of metal and semi-metal oxides in a vacuum of typically significantly better than 10E-5 Torr. Anti-reflection coating of plastic lenses in spinning drum coaters by means of sputter technology is a relatively recent development. A conventional vacuum system used for this purpose is shown in FIG. 1. The system includes a vacuum coating chamber 11, which contains a hollow workpiece holder or drum 12. Lens elements, such as lens 13 are arranged in columns on an external surface of the drum 12. A coating applicator 14 is located in a wall of the vacuum chamber adjacent the drum 12. In a preferred embodiment, the coating applicator may be a combination of magnetron sputtering targets, microwave plasma generator, reactive gas supply, and reversing power supply such as disclosed in U.S. Pat. No. 5,616,224 to Boling, which is hereby incorporated by reference.
A pumping plenum 15 is attached to vacuum pumps (not shown) which evacuate the vacuum chamber 12. A cryopumping surface is provided in the form of cryocoils 16 in the plenum 15. The cryopumping surface is also known as a "Meissner trap". Conventionally the Meissner trap takes the form of a coiled or serpentine metal tube through which a coolant passes. Advantageously, the coolant is maintained at a temperature well below the freezing point of water. The Meissner trap is used to remove water vapor from the system.
In most such drum coaters the placement of the cryocoils in the pumping chamber plenum is favored. The prime reason for this particular placement is that it is done with a view to protecting the subsequent pumps, especially large roughing pumps, from excessive water contamination which can reduce the life and efficacy of such pumps. A secondary reason for such placement appears to be the desire to keep the cryocoils away from the rotating drum to avoid somewhat greater mechanical complexity, the danger of the parts held on the drum colliding with the cryocoils and to provide greater ease of maintenance of both the coils and the drum. However, Meissner traps have been located in the vacuum chamber rather than the plenum in systems for vacuum coating work pieces on holders other than plastic lenses on drums as disclosed in U.S. Pat. No. 4,647,361 to Bauer and U.S. Pat. No. 5,121,707 to Kanoo.
Plastic lenses are usually thoroughly baked at temperatures in excess of 90.degree. C. for one to several hours prior to introduction to the vacuum system to reduce water vapor outgassing. Load sizes of plastic lenses have been limited to about 200. Pump-down times to base vacuums in the 10E-6 Torr regime are typically in the order of 30 minutes or more.
It is another object of the present invention to reduce the processing time required to deposit vacuum coatings on plastic parts.
It is another object of the present invention to provide an apparatus for depositing a high quality vacuum coating on large numbers of plastic lens elements in a system which is relatively inexpensive to construct and operate.
These and other objects and features of the present invention will be apparent from the written description and drawings presented herein.